This invention relates to plastic injection machines, and more particularly to pressure controls and methods for such a machine and a thrust bearing for transmitting the pressures generated by the injection machine.
Machines for pressure injecting hardenable plastic materials are well known in the art. Examples include U.S. Pat. Nos. 2,318,031, 2,734,226, and 3,436,793. These machines typically plasticize the plastic material and then inject the material under pressure into suitable molds. The injection machine commonly maintains the material in a pressurized state for a finite period of time to assure complete filling of the mold cavities and to prevent loss of size of the finished article due to shrinkage of the plastic material as it is cooled by contact with the walls of the mold.
Such machines have proven quite satisfactory in the injection molding of a wide variety of products. However, there still remain certain drawbacks. For example, the plastic must be injected very rapidly in order to eliminate cold wall freeze offs along the main runner, within the various mold cavities, and the cavity gates. Such rapid injection is accomplished by using very high pressures to drive the plastic into the mold at a high rate of speed.
These same high pressures frequently cause flashing of the mold as the plastic leaks between the mold halves under the high pressure. Extreme clamping forces must therefore be applied to the molds, not only to reduce flashing as far as possible, but also to keep the mold halves tightly adjacent one another. Further, when metal inserts such as wires, metal blades, pins, and so on are included within the mold, the high pressures not infrequently dislodge the inserts from the desired position within the mold. Molded inserts of this type are quite commonly employed in electrical, automotive, appliance, and other items, and the integrity and proper functioning of the molds and the injection machine are therefore of considerable importance.
Since the plastic material is oftentimes plasticized in the injection cylinder barrel by a screw which also serves to pressurize the plastic material during injection, appropriate bearing means must be provided for the screw to withstand the high forces of injection while enabling the screw to be rotated for recharging the machine.
In U.S. Pat. No. 3,436,793, assigned to the same assignee as this invention, there is described and claimed an arrangement by which the opening of the nozzle is somewhat delayed until the plasticized material in the barrel is substantially pressurized by the screw, to provide for the rapid filling and injection into the mold cavity. While the apparatus shown therein has been highly successful, it did not deal with the problem of high pressure flashing or the dislodgement of the metal parts and inserts, as mentioned above.